Fine structure and morphogenesis of the sclerite epicuticle in the Atlantic shore crab Carcinus maenas

Structure and mineralization of the spearing mantis shrimp (Stomatopoda; Lysiosquillina maculata) body and spike cuticles

Stomatopoda is a crustacean order including sophisticated predators called spearing and smashing mantis shrimps that are separated from the well-studied Eumalacotraca since the Devonian. The spearing mantis shrimp has developed a spiky dactyl capable of impaling fishes or crustaceans in a fraction of second. In this high velocity hunting technique, the spikes undergo an intense mechanical constraint to which their exoskeleton (or cuticle) has to be adapted. To better understand the spike cuticle internal architecture and composition, electron microscopy, X-ray microanalysis and Raman spectroscopy were used on the spikes of 7 individuals (collected in French Polynesia and Indonesia), but also on parts of the body cuticle that have less mechanical stress to bear. In the body cuticle, several specificities linked to the group were found, allowing to determine the basic structure from which the spike cuticle has evolved. Results also highlighted that the body cuticle of mantis shrimps could be a model close to the ancestral arthropod cuticle by the aspect of its biological layers (epi- and procuticle including exo- and endocuticle) as well as by the Ca-carbonate/phosphate mineral content of these layers. In contrast, the spike cuticle exhibits a deeply modified organization in four functional regions overprinted on the biological layers. Each of them has specific fibre arrangement or mineral content (fluorapatite, ACP or phosphate-rich Ca-carbonate) and is thought to assume specific mechanical roles, conferring appropriate properties on the entire spike. These results agree with an evolution of smashing mantis shrimps from primitive stabbing/spearing shrimps, and thus also allowed a better understanding of the structural modifications described in previous studies on the dactyl club of smashing mantis shrimps.

Comparative ultrastructure of cells and cuticle in the anterior chamber and papillate region of Porcellioscaber (Crustacea, Isopoda) hindgut

Isopod hindgut consists of two anatomical and functional parts, the anterior chamber, and the papillate region. This study provides a detailed ultrastructural comparison of epithelial cells in the anterior chamber and the papillate region with focus on cuticle ultrastructure, apical and basal plasma membrane labyrinths, and cell junctions. Na⁺/K⁺-ATPase activity in the hindgut epithelial cells was demonstrated by cytochemical localisation. The main difference in cuticle ultrastructure is in the thickness of epicuticle which is almost as thick as the procuticle in the papillate region and only about one sixth of the thickness of procuticle in the anterior chamber. The apical plasma membrane in both hindgut regions forms an apical plasma membrane labyrinth of cytoplasmic strands and extracellular spaces. In the papillate region the membranous infoldings are deeper and the extracellular spaces are wider. The basal plasma membrane is extensively infolded and associated with numerous mitochondria in the papillate region, while it forms relatively scarce basal infoldings in the anterior chamber. The junctional complex in both hindgut regions consists of adherens and septate junctions. Septate junctions are more extensive in the papillate region. Na⁺/K⁺-ATPase was located mostly in the apical plasma membranes in both hindgut regions. The ultrastructural features of hindgut cuticle are discussed in comparison to exoskeletal cuticle and to cuticles of other arthropod transporting epithelia from the perspective of their mechanical properties and permeability. The morphology of apical and basal plasma membranes and localisation of Na⁺/K⁺-ATPase are compared with other arthropod-transporting epithelia according to different functions of the anterior chamber and the papillate region.

Cuticle morphogenesis in crustacean embryonic and postembryonic stages

The crustacean cuticle is a chitin-based extracellular matrix, produced in general by epidermal cells and ectodermally derived epithelial cells of the digestive tract. Cuticle morphogenesis is an integrative part of embryonic and postembryonic development and it was studied in several groups of crustaceans, but mainly with a focus on one selected aspect of morphogenesis. Early studies were focused mainly on in vivo or histological observations of embryonic or larval molt cycles and more recently, some ultrastructural studies of the cuticle differentiation during development were performed. The aim of this paper is to review data on exoskeletal and gut cuticle formation during embryonic and postembryonic development in crustaceans, obtained in different developmental stages of different species and to bring together and discuss different aspects of cuticle morphogenesis, namely data on the morphology, ultrastructure, composition, connections to muscles and molt cycles in relation to cuticle differentiation. Based on the comparative evaluation of microscopic analyses of cuticle in crustacean embryonic and postembryonic stages, common principles of cuticle morphogenesis during development are discussed. Additional studies are suggested to further clarify this topic and to connect the new knowledge to related fields.

Exoskeletal cuticle differentiation during intramarsupial development of Porcellio scaber (Crustacea: Isopoda)

Exoskeletal crustacean cuticle is a calcified apical extracellular matrix of epidermal cells, illustrating the chitin-based organic scaffold for biomineralization. Studies of cuticle formation during molting reveal significant dynamics and complexity of the assembly processes, while cuticle formation during embryogenesis is poorly investigated. This study reveals in the terrestrial isopod Porcellio scaber, the ultrastructural organization of the differentiating precuticular matrices and exoskeletal cuticles during embryonic and larval intramarsupial development. The composition of the epidermal matrices was obtained by WGA lectin labelling and EDXS analysis. At least two precuticular matrices, consisting of loosely arranged material with overlying electron dense lamina, are secreted by the epidermis in the mid-stage embryo. The prehatching embryo is the earliest developmental stage with a cuticular matrix consisting of an epicuticle and a procuticle, displaying WGA binding and forming cuticular scales. In newly hatched marsupial larva manca, a new cuticle is formed and calcium sequestration in the cuticle is evident. Progression of larval development leads to the cuticle thickening, structural differentiation of cuticular layers and prominent cuticle calcification. Morphological characteristics of exoskeleton renewal in marsupial manca are described. Elaborated cuticle in marsupial larvae indicates the importance of the exoskeleton in protection and support of the larval body in the marsupium and during the release of larvae in the external environment.

Eco-morphological studies on pleopodal lungs and cuticle in Armadillidium species (Crustacea, Isopoda, Oniscidea)

Terrestrial isopods (Crustacea, Isopoda, Oniscidea) have adapted to land life by diverse morphological, physiological and behavioral changes. Woodlice species exhibit a large variety in this respect, their preferences ranging from moist to dry habitats. These moisture preference values are related to various morphological adaptations, rendering terrestrial isopods amenable to studying morphological adaptations to terrestrial life. We performed a comparison of four Armadillidium species (Armadillidium zenckeri, Armadillidium nasatum, Armadillidium versicolor, Armadillidium vulgare), by quantifying two morphological traits: the extent of the interfacial endothelium between the respiratory space and the hemolymph within pleopodal lungs and the thickness of tergite cuticle, which are 'key factors' in determining protection from desiccation. These values were measured from light micrographs of cross-sectioned lungs. The cosmopolitan A. vulgare, as a habitat generalist, seems to be the most resistant against desiccation and other environmental conditions, while A. zenckeri is the most sensitive one. Light microscopic studies revealed that the four species can be ordered similarly, if we compare them by the extension of the endothelial interface and cuticle thickness, suggesting that these morphological traits are important determinants of their distribution on habitat, microhabitat scales and through the existence of suitable habitats - together with many other factors - the geographical pattern of species occurence.

Preferred crystallographic texture of alpha-chitin as a microscopic and macroscopic design principle of the exoskeleton of the lobster Homarus americanus

The crystallographic texture of the crystalline alpha-chitin matrix in the biological composite material forming the exoskeleton of the lobster Homarus americanus has been determined using synchrotron X-ray pole figure measurements and the calculation of orientation distribution functions. The study has two objectives. The first one is to elucidate crystallographic building principles via the preferred synthesis of certain orientations in crystalline organic tissue. The second one is to study whether a general global design principle exists for the exoskeleton which uses preferred textures relative to the local coordinate system throughout the lobster cuticle. The first point, hence, pursues the question of the extent to which and why alpha-chitin reveals preferred textures in the lobster cuticle. The second point addresses the question of why and whether such preferred textures (and the resulting anisotropy) exist everywhere in the exoskeleton. Concerning the first aspect, a strong preference of a fiber texture of the orthorhombic alpha-chitin is observed which is characterized by a 020 crystal axis normal to the exoskeleton surface for the chitin matrix. The second question is tackled by studying samples from different parts of the carapace. While the first aspect takes a microscopic perspective at the basic structure of the biological composite, the second point aims at building a bridge between an understanding of the microstructure and the macroscopic nature of a larger biological construction. We observe that the texture is everywhere in the carapace optimized in such a way that the same crystallographic axis of the chitin matrix is parallel to the normal to the local tangent plane of the carapace. Notable differences in the texture are observed between hard mineralized parts on the one hand and soft membranous parts on the other. The study shows that the complex hierarchical microstructure of the arthropod cuticle can be well described by surprisingly simple crystallographic textures.

Formation of the hinge in the podocopan ostracode Loxoconcha pulchra

The hinge structure in the podocopan ostracode Loxconcha pulchra was examined throughout its molt cycle using ultrastructural and histological procedures. The structure consists of ligament and hingement, and develops along the attached margin of the right and left valves. In Stage C the hingement of both valves interdigitates beneath the ligament, and a series of outer epidermal cells (dorsal epidermal cells), exhibiting abundant granules, underlie the hinge structure. Apolysis occurs at Stage D1, and electron-dense granular materials of variable diameter are seen within the ecdysial space. Epicuticle formation begins at Stage D2 and is complete before Stage D4. In Stage D2 the new epicuticle appears as a dotted line consisting of numerous grain-like materials. The dorsal epidermal cells, which actively secrete the numerous granules during molting, increase their size and reveal the electron-dense substances in the cytoplasm from Stage D2. At early Stage D3 the procuticle deposition of ligament commences inside the epicuticle, and is completed in Stage D4. In Stage D4 the uncalcified procuticle is secreted under the whole area of carapace, and the new carapace is then ready for ecdysis. After ecdysis, calcification of the carapace commences from the dorsal and ventral marginal areas towards the central area. During Stage A there is no further cuticle deposition in the ligament, although the dorsal epidermal cells secrete as actively in the postmolt stage as in premolt. The dorsal epidermal cells begin to form the hingement just after ecdysis. Cuticle deposition of the hingement proceeds asynchronously in the two valves: the hingement of the right valve is formed prior to that of left one in L. pulchra. The right hingement functions as a mold for the left hingement to form the precise interdigitated structure in L. pulchra. These observations suggest that the ostracode ligament is a unique cuticle, which should not be confused with the cuticles of other arthropods. The work establishes, for the first time, a description of the formation of the hingement in podocopan ostracodes.

Early pattern of calcification in the dorsal carapace of the blue crab,Callinectes sapidus

The pattern of calcium carbonate deposition was observed in the dorsal carapace of premolt (D2-D3) and early postmolt (0-48 h) blue crabs, Callinectes sapidus, using scanning (SEM) and transmission (TEM) electron microscopy. Samples of dorsal carapace for SEM were quick-frozen in liquid nitrogen, subsequently lyophilized, and viewed using secondary and backscattered electrons as well as X-ray maps of calcium. Pieces of lyophilized cuticle were also embedded in epoxy resin and subsequently sectioned and viewed with TEM and SEM. Fresh pieces of dorsal carapace for TEM were also fixed in 2.5% glutaraldehyde in phosphate buffer followed by postfixation in 1% OsO4 in cacodylate buffer. Calcium concentrations were determined using atomic absorption spectrophotometry and quantitative X-ray microanalysis. Calcium accumulation began in the cuticle at 3 h postmolt at the epicuticle/exocuticle boundary and at the distal and proximal margins of the interprismatic septa (IPS). The bidirectional calcification of the IPS continued until the two fronts met at 5-8 h postmolt. The roughly hexagonal walls of the IPS formed a honeycomb-like structure that resulted in a rigid cuticle. The walls of the canal containing sensory neurons also calcified at 3 h, thereby imparting rigidity to the structure and additional strength to the cuticle. Examination of thin sections of lyophilized cuticle and fixed cuticle revealed that the first mineral deposited is more soluble than calcite and is probably amorphous calcium carbonate. The amorphous calcium carbonate is transformed to calcite along a front that follows the original deposition and is probably controlled by a specialized matrix within the IPS. Since amorphous calcium carbonate is isotropic, it would also make the mineral in the exocuticle stronger by an equal distribution of mechanical stress.

Formation of the arthrodial membrane in the blue crab, Callinectes sapidus

In this study the pattern of arthrodial membrane deposition in Callinectes sapidus was determined by histological and ultrastructural examination of tissues from the carpus joint of the cheliped collected during premolt, ecdysis, postmolt, and intermolt. Apolysis in the arthrodial membrane occurs at stage D(0) and is synchronous with apolysis of the calcified cuticle. Epicuticle formation begins at early stage D(1) and is completed in late stage D(1). Procuticle deposition starts at D(2) and continues until ecdysis. Numerous cytoplasmic extensions occur throughout the lamellae. Component fibers of the arthrodial membrane are intimately associated with dense plaques on the apical membrane of the underlying hypodermal cells, suggesting a site for fiber polymerization. Deposition of the arthrodial membrane continues after ecdysis, with most of the cuticle thickening occurring during stage C. When stained with PAS and counterstained with hematoxylin, a difference can be discerned between preecdysial and postecdysial procuticle of the arthrodial membrane, a distinction not made in previous studies. The boundary between the arthrodial membrane and calcified cuticle is thicker than either of the two layers and the layers overlap rather than butting up against one another. This pattern suggests that underlying hypodermal cells have to produce multiple types of cuticle over the molt cycle. A summary of the various molting patterns in C. sapidus suggests that the control of these diverse events may prove to be complex.

Glycoproteins from the cuticle of the Atlantic shore crab Carcinus maenas: I. Electrophoresis and Western-blot analysis by use of lectins

The protein and glycoprotein content of four different neutral or acidic solvent extracts (0.5 M KCl, 10% EDTA, 0.1 N HCl, or 2% acetic acid) from the mineralized exoskeleton of a decapod crustacean, the Atlantic shore crab Carcinus maenas, were characterized by quantitative analysis of proteins, SDS-PAGE analysis, and probing with lectins on blots. The lectins used were Conconavalin A, Jacalin, soybean agglutinin, Maackia amurensis agglutinin II, and Sambucus nigra agglutinin. The results show that many proteins can be obtained from the crab cuticle without strong denaturants in the extraction medium. Many of the extracted cuticle proteins appeared to be glycosylated, bearing O-linked oligosaccharides and N-linked mannose-rich glycans. N-acetyl-galactosamine and N-acetylneuraminic acids were revealed, for the first time, as terminal residues on N-linked mannose-rich structures of crab cuticle glycoproteins. Sialylated glycoproteins might thus be involved in organic-mineral interactions in the calcified crab exoskeleton. The amount and variety of glycoproteins extracted with the acidic solvents are obviously different from those extracted with neutral solvents. HCl proved to be the best of the tested extraction solvents and a valuable alternative to EDTA.

"Exolysosomes," enzyme-containing vesicles in the ecdysial space of molting crabs

Free vesicle-like bodies (VLBs) present in the ecdysial space of cuticle regions undergoing degradation during preecdysis of the Atlantic shore crab Carcinus maenas have been interpreted either as infectious organisms or as secretion structures associated with degradation of the old cuticle. Ultrastructural, cytochemical, and immunocytological investigations were performed to test these hypotheses and to see whether VLBs are peculiar to this crab species. Similar VLBs were systematically found in two other preecdysial crabs, Cancer pagurus and Macropipus puber. In Car, maenas, they originate during early premolt inside Golgi buddings and are often gathered into large vacuoles in epidermal cells. The histochemical azo-dye technique and a cerium-based cytochemical method revealed acid phosphatase activity in both the ecdysial space and the VLBs, while Feulgen's method and immunocytological labeling always failed to reveal any DNA or RNA in either the ecdysial space or the VLBs. We conclude that VLBs are not infectious organisms but "extracellular" cuticle-degrading organelles of lysosomal origin and propose to coin them "exolysosomes."